1. Field of the Invention
This invention relates to an improvement on a tip optical element for immersion objectives of a high resolving power, used in a microscope.
2. Description of Related Art
In order to obtain a high resolving power and a high magnification, immersion objectives have been used in such a way that space between a specimen and a front lens (a tip optical element) of a microscope objective is charged with a liquid, such as water or oil, which has a higher refractive index than in air. In the front lens, a hemispherical or nearly hemispherical lens is used, and in order to improve correction for aberration, a minute lens which is different in dispersion or refractive index from the front lens is often embedded in the middle portion of the tip of the lens (refer to FIG. 2 of Japanese Utility Model Kokai No. Hei 4-46416, for instance).
An objective using such a front lens, as shown in FIG. 1 (corresponding to FIG. 5 of the above publication), has the problem that a detrimental ray of light 60 is incident on the front lens 90 from outside the effective diameter thereof to produce flare. In order to solve this problem, some methods of blocking the detrimental ray 60 incident from outside the effective diameter of the front lens 90 have been proposed. For example, there are, as shown in FIG. 2, a method of cementing a stop 30 to a frame 70 holding the front lens 90 to block the detrimental ray 60; as in FIG. 3, a method of providing a flange 40 to the lens holding frame 70 and holding the front lens 90 by caulking or cementation to block the detrimental ray 60 with this flange 40; and as in FIG. 4, a method of evaporating a metal film 50 outside the effective diameter of a beam of light on the surface of the front lens 90 to block the detrimental ray 60.
In the evaporation of metal film 50, the lens must be treated at temperatures of 200–300° C. When the minute lens is embedded in and cemented to the front lens 90, the embedded lens sometimes comes loose due to such high temperatures. Usually, the minute lens, after being embedded (see FIG. 5A), is ground so that an end surface A of the minute lens is flush with an end surface B of the lens in which the minute lens is embedded (see FIG. 5B). This is because if there is a step on the surface of the tip of the objective, air bubbles will easily enter into oil or water with which the space between the tip of the objective and the specimen is charged. Consequently, it is not proper that the metal film 50 is evaporated before the minute lens is embedded.